there is no ultimate rod unless they just made one for platinum, but there is a super rod which is the best rod in the game. to get that you need to get the national dex then go to the fight area and go to the route the doesn't have water. before you go into the thing that brings you into the actual route there is a fisherman and he gives you it.
Old Rod.
In Pokemon Platinum the Good Rod is to the right of Hearthome City. The fisherman on Route 209 will give you the good rod.
You get the old rod to the left of jubilife city.
a good rod or super rod
On Pokemon Platinum, Carvanha can be caught in the Great Marsh.PLEASE NOTE: REQUIRES SUPER ROD.
route 218 (good rod)
fish with the super rod at the pokemon league
You can receive a Super Rod from a fisherman in the Fight Area on Pokemon Platinum, Diamond and Pearl.
At route 209 from a fisherman.
Use a super rod.
yes
fisherman on route 209